a. Field of the Invention
This invention pertains to optical, reflector antennas. More particularly this invention pertains to the scanning of the beam from a reflector antenna by using a subreflector that is offset from the optical axis of the reflector antenna and revolves about that axis.
b. Description of the Prior Art
Many optical, radio-frequency antennas, such as the cassegrainian antenna depicted in FIG. 1 utilize a feed 11, a main reflector 12 and a subreflector 13 to generate a narrow, pencil-shaped beam aligned with the centerline or optical axis 14 of the antenna. Such antennas are used for many purposes such as communicating with, and for the tracking of, earth satellites. However, in instances where the initial position of the satellite is not well known, the antenna must first search for and find the position of the satellite in order to "lock-on to" or acquire and begin tracking the satellite. Unfortunately, the narrow, pencil-shaped beam that is generated by such an antenna, and that has a fixed position relative to the main reflector, is not a desirable beam shape to use for the purpose of searching for and acquiring a satellite.
If the subreflector 13 that is depicted in FIG. 1 as being located on the centerline 14 of the antenna is modified so as to be offset from the centerline 14 by a small amount and is then caused to revolve about the centerline, this movement of the subreflector will, in turn, cause the center of the antenna pencil beam that is generated by the modified antenna also to be offset slightly from the centerline or optical axis 14 of the antenna and to move or scan along a circular path about the centerline of the antenna. The scanning motion of the beam that is generated by the modified antenna facilitates the search for and acquisition of a satellite.